THERE'S MORE than meets the eye to Israel's and Lebanon's agreement on an agenda for talks. For the agenda is not just a list of things to discuss: security, future relations, Israeli withdrawal. It's the outline of a prospective settlement. Merely to inscribe on the agenda, for instance, "termination of the state of war" is to register a commitment--in this instance, Lebanon's--to a goal the Israelis have sought for 30-odd years. To inscribe "evacuation of all foreign forces" commits Israel to a full withdrawal. In a mere three weeks, the two countries bargained out at least in principle their very large substantive difference over the degree of explicitness-- Israel wanting more, Lebanon less--that their relations should have after the troops depart.

How was this result achieved? The question is well worth asking in view of the regular invocations of Israeli "intransigence" and, to a lesser extent, Lebanese political frailty, qualities that are supposed to make progress next to impossible. An answer may also shed some light on the prospects of nailing down the all-important details from this point on.

First, on the Lebanese side: no one would call the Lebanese government strong. It is especially troublesome that President Gemayel either does not control or tends to indulge the Phalangist military forces built up by his late brother. The command of these forces, the country's largest, has its own line to the Israelis--specifically, it seems, to Defense Minister Ariel Sharon.

From Lebanon's summer ordeal, nonetheless, there did emerge a new degree of national feeling and a new degree of readiness on the part of other Arab states to stop creating trouble in Lebanon. Outside powers, moreover, have means at their disposal to strengthen the Lebanese central authority: they can help remove those foreign forces, build up the Lebanese army and supply peacekeeping units. So Lebanon is not immobile.

Israelis have their own reasons for moving ahead, not least to demonstrate that it pays for Arabs to sit down with them to negotiate. But a more pressing reason is the intense and growing unpopularity of the occupation. Continuing casualties and the strife among Lebanese are spreading the sickening sense of being dragged into an ugly swamp. It is entirely possible that many, if not most, of the government's own ministers might prefer to do what the polls indicate the public definitely wants to do: to withdraw immediately, at least to the border zone, the securing of which was the original stated objective of the summer invasion. Prime Minister Begin and his key ministers, aware that 90 percent of Israel's casualties were taken by going beyond this zone, are resisting these pressures. But they cannot ignore them.

Nor can it be ignored that many Israelis, including Mr. Begin, have long favored hooking up on a more permanent basis with Lebanese Maronites and in effect partitioning Lebanon with Syria. Presumably it is to keep open this option that Mr. Sharon, who is himself fighting for political survival, is playing the Phalangist card. But the basic political fact remains that the popular will is on the side of getting out of the Lebanese "trap."

Reinforcing domestic pressures on Israel to negotiate a withdrawal is Ronald Reagan's impatience, which becomes known to the Israelis directly and through leaks. Unquestionably, the president feels an urgency to see Lebanon freed of all its occupiers. There is reason to believe, moreover, that Syria's withdrawal could be arranged: an Arab consensus supports and demands it, and the beset Damascus regime has its own reasons to end its bitter frustrations in Lebanon. Recent word that the Syrians are preparing to deploy improved (but 20-year-old) Soviet anti-aircraft missiles deep inside Syria sparked a flash of concern but would seem to be consistent with withdrawal.

The critical question may well be how President Reagan connects Lebanon and his Sept. 1 Palestinian initiative. Our hunch--and hope--is that he is serious about Sept. 1. Certainly, notwithstanding Israel's rejection of his call for a West Bank settlements freeze, he is still trying hard to pull King Hussein to the bargaining table. But whatever happens in that effort, there is no reason for dragging out the Lebanon talks. Now that the central question of the nature of post-withdrawal relations has been successfully treated, it shouldn't take all that much time to finish the job.